1. Field of the Invention
The present invention relates to a method and apparatus for ink droplet ejection in printers and, more particularly, to a method and apparatus for ink droplet ejection which uses ink-nonemission pulse for each print instruction.
2. Description of Related Art
Among nonimpact-type printers which have been employed in place of impact-type printers recently, there are ink jet printers featuring simplicity in operating principle and easy applicability to multiple gradation and colorization. Particularly, drop-on-demand ink jet printers designed to eject ink droplets only at printing have rapidly become prevalent owing to high efficiency of ink ejection and low running cost.
In JP-A-63-247051, there is disclosed a drop-on-demand ink ejection apparatus of a shear mode type formed with piezoelectric material. As shown in FIGS. 12 and 13, an ink droplet ejection apparatus 600 of this type comprises a base wall 601, a top wall 602 and shear-mode actuator walls 603. Each actuator wall 603 comprises a lower wall 607 made of piezoelectric material, which is bonded to the base wall 601 and polarized in arrow direction 611, and an upper wall 605 made of piezoelectric material, which is bonded to the top wall 602 and polarized in arrow direction 609. Two actuator walls 603 are arranged in a pair to provide an ink channel 613 therebetween, and a space 615 narrower than the ink channel 613 is provided between adjacent pairs of actuator walls 603.
A nozzle plate 617 having a nozzle 618 is secured at one end of each ink channel 613, and an ink supply source (not shown) is connected at the other end thereof. Electrodes 619 and 621 are provided as metallized layers on both sides of each actuator wall 603. More specifically, the electrode 619 is provided on the actuator wall 603 forming the ink channel 613, and the electrode 621 is provided on the actuator wall 603 forming the space 615. The surface of the electrode 619 is coated with an insulating layer 630 for insulation against ink. The electrode 621 having the space 615 therein is connected with the ground 623, and the electrode 619 having the ink channel 613 therein is connected with a control device 625 which applies actuator drive signals.
In operation, the control device 625 applies a drive signal to each ink channel 613 to cause piezoelectric thickness slide deformation of each actuator wall 603 so that a volume of the ink channel 613 is increased. For instance, as shown in FIG. 14, when the drive signal having a voltage amplitude E (V) is applied to an electrode 619c of one ink channel 613c, electric fields are produced in actuator walls 603e and 603f in arrow directions 631 and 632 respectively, causing piezoelectric thickness slide deformation of the actuator walls 603e and 603f to occur to increase a volume of the ink channel 613c. In this operation, pressure in the ink channel 613c including a vicinal part of a nozzle 618c is decreased. Application of the voltage E (V) is maintained during a period of one-way propagation time T of the pressure wave in the ink channel 613c, thereby causing an ink supply source (not shown) to feed ink thereinto.
The one-way propagation time T indicates a period of time required for a pressure wave in the ink channel 613c to complete propagation in the longitudinal direction of the ink channel 613c. Using length `L` of the ink channel 613c and acoustic velocity `a` in ink in the ink channel 613c, `T` is expressed as T=L/a.
On the principle of pressure wave propagation, after a lapse of time T following application of the voltage, pressure in the ink channel 613c is reversed to become positive pressure. At timing of pressure reversal, voltage being applied to an electrode 621c of the ink channel 613c is reset to zero (0) V. Thus, the actuator walls 603e and 603f are restored to normal (as shown in FIGS. 12 and 13), applying pressure to ink. At this time, the positive pressure is added to pressure which has been produced by restoration of the actuator walls 603e and 603f to normal so that relatively high pressure is generated in the vicinity of the nozzle 618c in the ink channel 613c, thereby ejecting a droplet of ink through the nozzle 618c.
In this conventional ink droplet ejection apparatus 600, since a volume of ink per droplet to be ejected is determined depending on such factors as configuration of the ink channel 613, drive signal voltage amplitude (E), etc., it is required to alter the configuration of the ink channel if the amount of ink per droplet must be increased to provide better quality of printing. Still more, even if the necessary amount of ink per droplet is attained, it is required to lower the drive frequency of the drive signal for coping with degradation of stability in ink droplet ejection.
It is also known, as shown in FIG. 15 to generate two ink-emission pulses A and B successively in each drive signal in response to a one-dot print instruction so that two successive ink droplets on the fly are combined into a single droplet having a relatively large volume before an ink droplet formed by the preceding emission pulse A separates completely from ink in the ink channel. However, in such an arrangement, pressure in the vicinity of the nozzle 618 becomes extremely high due to the second emission pulse B, making it difficult to attenuate pressure readily. As in the foregoing case, even if the necessary amount of ink per droplet is attained, the drive frequency of the drive signal must be decreased to cope with degradation of stability in ink droplet ejection.
Further, if viscosity of ink decreases due to an increase in temperature or any other cause, residual pressure wave oscillation may cause an undesired accidental droplet of ink to be ejected after ejection of a single droplet or plural droplets, making it difficult to attain satisfactory quality of printing.
In another known arrangement, disclosed in JP-A-62-299343 for example, an ink-emission pulse for ink droplet ejection is followed by a cancel pulse to reduce residual pressure wave oscillation in an ink channel. More specifically, a pressure wave for ink droplet ejection rebounds from the front and rear ends of the ink channel, and a nozzle meniscus is vibrated after a lapse of time 4 T following the start of ink droplet ejection. To obviate this phenomenon, a pressure wave for phase reversal is produced. However, in such an arrangement that a cancel pulse is generated after a lapse of time 4 T following the start of ink droplet ejection, it is impossible to use a plurality of successive emission pulses. Furthermore, in addition to a positive power supply for generating ink-emission pulses, a negative power supply for generating reverse-phased cancel pulses is required causing disadvantages of complexity in the control device circuit and an increase in production cost.